The invention is related to optical amplifiers, more particularly, to rare-earth doped phosphate optical fibers for amplifiers pumped by diode or solid state laser sources. The rare-earth ions, erbium and ytterbium, are co-doped at high levels to provide for extremely high gain. Erbium ion concentration is increased to provide very high active ion content. Ytterbium ion concentration is increased to provide efficient absorption of pump power. The phosphate glass fiber provides for a low-loss and stable fiber host. Phosphate glass fibers can be produced with a temperature coefficient of refractive index close to zero.
Deregulation, long distance price declines, bandwidth stockpiling, and internet usage are driving bandwidth demand in telecom and datacom networks. Data traffic is now growing at 100 percent or more per annum, straining global fiber capacity. Dense Wavelength Division Multiplexing (DWDM), where multiple wavelength channels propagate within a single fiber multiplies fiber capacity by 2-128 times or more, is an approach for extending carrier capacity without the need of deploying new fiber. Systems being deployed today generally can transmit from 8 up to 128 channels in the 1550 nm low-dispersion window. Channel spacing ranges from 1.6 nm (200 GHz) to 0.4 nm (50 GHz).
Optical amplifiers are considered enabling components for bandwidth expansion in DWDM fiber optic communications systems. In particular, silica glass Erbium Doped Fiber Amplifiers (EDFA) exhibit many desirable attributes including high gain, low noise, negligible crosstalk and intermodulation distortion, bit-rate transparency, and polarization insensitive gain. These properties make optical fiber amplifiers superior to semiconductor devices as amplifiers in fiber optic systems. Moreover, fiber-based amplifiers do not require conversion from electrical energy to photon energy as do semiconductor devices. In a communications system of any significant size, there is typically a distribution network that includes long communication paths and nodes where the network branches. In such a network, amplifiers are required in order to maintain the amplitude of the signal and the integrity of any data in route between a source and destination. For these amplifiers to function properly, the amplifiers must exhibit high small signal gains and/or high output saturation powers.
Application of erbium-doped optical fibers as amplifiers has received considerable attention recently because the characteristic gain bandwidth of these fibers is within the telecommunications window of 1.5 xcexcm commonly used in fiber optic communications systems. Since the announcement of a single mode Er3+doped fiber amplifier (EDFA) in 1987 at the University of Southampton, enormous research has been performed, and more than 400 U.S. patents have been issued in fiber amplifiers. To date, all erbium fiber amplifiers use erbium doped silica fibers more than one meter long to achieve greater than 20 dB gain near the 1.54 xcexcm range. More commonly, the length of the erbium doped silica fiber is approximately 10 to 20 meters. Such lengths are not practical for assembly into integrated optical components. There is a compelling need for amplifiers that can introduce high gain into an integrated, compact package.
To shorten length in fiber amplifiers, high gain must be achieved. In order to enable fiber amplifiers of only a few centimeters in length, magnitudes of doping two orders higher than what is commercially achievable (xcx9c1018 cmxe2x88x923) in silica fiber amplifiers is required. However, in silica fiber, cooperative upconversion and ion clustering effects develop from the interactions between nearby ions in silica glass, and electrons depopulate from the erbium metastable level (4I13/2). Thus, increased doping in silica glass does not improve gain.
Other glasses such as for example phosphate glasses exhibit high solubility and large emission cross sections for many rare-earth ions. Phosphate glasses for optical components have been investigated. Y. L. Lu, Y. et al., in xe2x80x9cFluorescence and attenuation properties of Er+3-doped phosphate glass fibers and efficient infrared-to-visible up-conversion,xe2x80x9d Applied Physics B, Vol. 62, pp.287-291 (1996) and Ya Lin Lu et al., in xe2x80x9cProperties of Er+3 doped phosphate glasses and glass fibers and efficient infrared to visible upconversion,xe2x80x9d Journal of Materials Science, Vol. 30, No. 22, Nov. 15, 1995, pp.5705-10, (1995) discuss phosphate glass fiber for use in up-conversion schemes. S. Jiang et al., in xe2x80x9cEr+3 doped phosphate glasses and lasers,xe2x80x9d Journal of Non Crystalline Solids, Vol.239, No. 1-3, October 1998, pp. 143-8, show phosphate glasses for application as bulk lasers. T. Nishi et al., in xe2x80x9cThe amplification properties of a highly Er+3 doped phosphate fiber,xe2x80x9d Jpn. J Appl. Phys., Vol. 31 (1992), Pt. 2, 2B, pp. L177-L179, show phosphate fiber with moderate erbium oxide doping. The maximum gain per unit length reported by Nishi et al. was only 1 dB/cm. S. Jiang, T. Luo et al. in xe2x80x9cNew Er 3+ doped phosphate glass for ion-exchanged waveguide amplifiers,xe2x80x9d Optical Engineering, Vol. 37, No. 12, December 1998, pp. 3282-6, disclose phosphate glasses for application in ion-exchanged waveguide amplifiers.
In addition, a number of patents have addressed doped glasses in various optical applications. For example, Hsu et al. (U.S. Pat. No. 5,425,039), Myers (U.S. Pat. No. 4,962,067), Myers et al. (U.S. Pat. No. 4,333,848), Myers et al. (U.S. Pat. No. 4,248,732), Myers et al. (U.S. Pat. No. 4,075,120), each disclose doped fibers for application as fiber lasers. In addition, Myers et al. (U.S. Pat. No. 5,322,820) and Myers (U.S. Pat. No. 5,164,343) disclose various glass compositions for laser applications. Grubb et al. (U.S. Pat. No. 5,225,925) disclose silica fibers or phosphorous doped silica fiber. Andrews et al. (U.S. Pat. No. 4,962,995) disclose glasses that are optimized for pumping by 800 nm laser light.
Recently, Y. C. Yan et al., in xe2x80x9cNet optical gain at 1.53 xcexcm in an Er-doped phosphate glass waveguide on silicon,xe2x80x9d Optical Amplifiers and Their Applications, Topical Meeting. OSA Trends in Optics and Photonics Series, Vol. 16. Opt. Soc. America, Washington, D.C., USA; 1997; xlv+526, pp.93-5, investigated doped phosphate glasses as a high gain medium for planar waveguide amplifiers at wavelength of 1.5 xcexcm. Y. C. Yan et al., in xe2x80x9cErbium-doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 xcexcm,xe2x80x9d Applied Physics Letters, Vol.71, No. 20, Nov. 17, 1997, pp. 2922-4 reported a gain of 4.1 dB in a 1 cm long phosphate glass waveguide prepared by an R-F sputtering technique. D. Barbier et al., in xe2x80x9cNet gain of 27 dB with a 8.6-cm-long Er/Yb-doped glass-planar-amplifier,xe2x80x9d OFC ""98 Optical Fiber Communication Conference and Exhibit, Technical Digest, Conference Edition 1998 OSA Technical Digest Series Vol.2 (IEEE Cat. No.98CH36177), Opt. Soc. America, Washington, D.C., USA; 1998; vii+421, pp.45-6, demonstrated a net gain of 27 dB in a 8.6 cm long ion-exchanged Er/Yb-doped phosphate glass waveguide.
Despite the high gains achieved for example in phosphate glass waveguides, planar waveguide amplifiers have significant disadvantages when compared with fiber amplifiers including polarization sensitivity, optical mode mismatch between waveguides and fiber networks, large propagation losses, and complicated fabrication processes.
One object of the invention is to provide a phosphate glass optical fiber amplifier with a gain per unit length, greater than 1.5 dB/cm and preferable over 3 dB/cm.
Another object of the invention is to provide a high gain per unit length doped phosphate glass fiber which can be utilized as a fiber amplifier in an optical communications system.
Another object of the invention is to provide an erbium and ytterbium codoped phosphate glass fiber with high (concentrations well above concentrations deemed practical by current wisdom) erbium and ytterbium co-doping concentrations for high gain amplification within a short length of the optical fiber. A short-length optical fiber amplifier utilizing the high gain, short-length fiber is compatible with V-groove and micro-machining fabrication processes, making the short-length fibers compatible and integratable into optical component modules.
Still a further object of the invention is to provide a phosphate glass fiber with a core containing erbium (as Er2O3) and ytterbium (as Yb2O3) and a phosphate glass clad without erbium or ytterbium, wherein the phosphate glass fiber is manufactured using a rod-in-tube technique.
Another object of the invention is the application of the phosphate fiber in an optical amplifier, preferably in an integrated amplified or lossless splitter module, wherein a system signal is amplified (with the assistance of a pump laser diode) and fed to a splitter. The fiber may be only a few centimeters in length but exhibits a gain coefficient greater than 3 dB/cm at 1.54 microns.
Yet, another object of the invention is the application of the phosphate fiber in an optical amplifier, preferably in an integrated amplified or lossless combiner module, wherein a system signal is amplified (with the assistance of a pump laser diode) and fed to a combiner. The fiber may be only a few centimeters in length but exhibits a gain coefficient greater than 3 dB/cm at 1.54 microns.
A further object of the invention is the application of the phosphate fiber in an optical amplifier, preferably in an integrated amplified or lossless arrayed waveguide grating module, wherein the system signal channels are amplified (with the assistance of a pump laser diode) and fed to an arrayed waveguide grating. The fiber may be only a few centimeters in length but exhibits a gain coefficient greater than 3 dB/cm at 1.54 microns.
Still a further object of the invention is the application of the phosphate fiber in an optical amplifier, preferably in an integrated amplified or lossless modulator module, wherein the system signals are amplified (with the assistance of a pump laser diode) and fed to a Lithium Niobate optical modulator. The fiber may be only a few centimeters in length but exhibits a gain coefficient greater than 3 dB/cm at 1.54 microns.
Still another object of the invention to provide an efficient, long-lived erbium and ytterbium glass optical amplifier that is generally suitable for a variety of components in metro and local network applications, specifically in the area of fiber optic communication networks.
As such, one object of the invention is to provide doped phosphate glasses with a temperature coefficient of refractive index close to zero.
Another object is to provide an array of doped phosphate glass fibers mounted in a groove on a substrate such as for example a V-groove, where the array is pumped by a multi-mode laser diode bar orthogonal to the array.
Another object of the invention is to provide an erbium doped fiber with a large light-guiding region (diameter of the core ranging from 50 to 300 xcexcm, which is much larger than in erbium doped fibers deemed practical by current wisdom). The fiber may be only a few centimeters in length and is pumped with one or more high-power multi-mode 980 nm light-emitting laser diodes, each having an emitting cross-sectional area on the order of 1 xcexcmxc3x97100 xcexcm.
These and other objects are achieved according to a fiber amplifier of the present invention utilizing a phosphate glass optical fiber highly doped with rare-earth ions such as erbium, and preferably co-doped with ytterbium to enhance gain. The phosphate glass optical fibers exhibit high gain per unit length, enabling the use of short fiber strands to achieve the needed gain in practical fiber optical communication networks.
According to one aspect of the present invention, the high-gain phosphate optical glass fiber amplifiers are integrated onto substrates, such as in grooved substrates, to form an integrated optics amplifier module. An optical pump such as a semiconductor laser of suitable wavelength is used to promote gain inversion of erbium ionic metastable states and ultimately provide power amplification of a given input signal.
According to another aspect of the present invention, a phosphate fiber amplifier is integrated with other components such as splitters, combiners, modulators, or arrayed waveguide gratings to form lossless or amplified components that do not suffer from insertion loss when added to an optical network.
According to a further aspect of the present invention, the fiber amplifier includes a single fiber or an array of fibers. Further, the phosphate glass fibers are designed with a temperature coefficient of refractive index close to zero enabling proper mode performance as ambient temperatures or induced heating changes the temperature of the phosphate glass fiber. Fiber core diameters from standard sizes such as for example 5 xcexcm to large core sizes such as for example 50-100 xcexcm fibers are used for fiber amplifiers in the present invention.
According to one aspect of the present invention, it is recognized that erbium doped glass fibers, containing erbium concentrations far beyond the generally accepted optimum concentration for erbium ions in silica fiber, show fiber amplification in a short length and thus enable production of integrated high gain optical components.
According to a further aspect of the present invention, the erbium doped glass fibers are co-doped with ytterbium to enhance pumping of the erbium metastable levels and the resultant gain in the phosphate fibers.
Further, according to the present invention, there is provided a novel phosphate glass composition including the following ingredients by weight percentages: P2O5 from 30 to 80 percent, Yb2O3 from 0 to 12 percent, Er2O3 from 2.5 to 12 percent, R2O from 0 to 5 percent L2O3 from 5 to 30 percent, MO from 5 to 30 percent, where the sum of the weight percentages of Yb2O3 and Er2O3 is 2.5% or greater, R2O is selected from the alkali metal oxide group consisting of Li2O, K2O, Na2O, Rb2O, and mixtures thereof, MO is selected from the alkaline earth oxide group consisting of BaO, BeO, MgO, SrO, CaO, ZnO, PbO and mixtures thereof, and L2O3 is selected from the transition metal oxide group consisting of Al2O3, B2O3.Y2O3, La2O3, and mixtures thereof.